The preferred application of the present invention is in association with an analog cellular telephone, and thus, the cellular telephone field is the technological field most pertinent to the preferred embodiments of the invention. However, the present invention may also be employed in association with a dual mode (analog/digital) cellular telephone and like receiving devices. Accordingly, except as they may be expressly so limited, the scope of protection of the claims appearing at the end of this specification is not limited to applications of the invention involving an analog cellular telephone.
FIG. 1 is a block diagram of a cellular telephone. The cellular telephone includes a radio transceiver 10, a demodulator 12, an error correction decoder 16, and a voice decoder 18, which are all coupled to the speaker portion of a handset 20. (FIG. 1 also depicts a bidirectional equalizer 14 that is not relevant to the present invention.) The system further comprises, coupled to the microphone portion of the handset 20, a voice encoder 22, error correction encoder 24 and modulator 26.
The cellular telephone operates in the environment of a cellular system. A cellular system typically includes many cell sites and a centrally-located cellular switch, called a Mobile Telephone Switching Office (MTSO). Cell sites are usually spaced at distances of one-half to twenty miles and comprise one or more antennas mounted on a triangular platform placed on a tower or atop a tall building. The fundamental idea behind a cellular system is frequency reuse. This concept of frequency reuse is implemented by employing a pattern of overlapping cells, with each cell conceptually viewed as a hexagon. Frequency reuse allows the cellular system to employ a limited number of radio channels to serve many users. For example, a given geographic area may be served by N cells, divided into two clusters. Each cluster would contain N/2 cells. A separate set of channels would be assigned to each cell in a cluster. However, the sets used in one cluster would be reassigned in the other cluster, thus reusing the available spectrum. The signals radiated from a cell in channels assigned to that cell would be powerful enough to provide a usable signal to a mobile cellular telephone within that cell, but preferably not powerful enough to interfere with co-channel signals in distant cells. All cellular telephones within the system would preferably be capable of tuning to any of the channels.
The Federal Communications Commission (FCC) has allocated a 25 MHz spectrum for use by cellular systems. This spectrum is divided into two 12.5 MHz bands, one of which is available to wire line common carriers only and the other of which is available to non-wire line common carriers only. In any given system, the non-wire line service provider operates within the "A side" of the spectrum and the wire line provider operates within the "B side" of the spectrum. Cellular channels are 30 KHz wide and include control channels and voice channels. Each cell site (or, where a cell site is sectored, each sector of that cell site) uses only a single control channel. The control channel from a cell site to a mobile unit is called the "forward" control channel and the control channel from the cellular telephone to the cell site is called the "reverse" control channel. Signals are continuously broadcast over a forward control channel by each cell site.
When a cellular telephone is first turned on, it scans all forward control channels, listening for the channel with the strongest signal. The telephone then selects the forward control channel with the strongest signal and listens for system overhead messages that are broadcast periodically, for example, every 0.8 seconds. These overhead messages contain information regarding the access parameters to the cellular system. The overhead messages also contain busy/idle bits that provide information about the current availability of the reverse control channel for that cell. When the reverse control channel becomes free, as indicated by the busy/idle bits, the cellular telephone attempts to register itself with the system by seizing the reverse control channel.
Cellular telephones, while in an idle or standby mode, must constantly monitor a continuous stream of data messages sent by a cell site over a forward control channel. The format of these messages is depicted in FIG. 2 and is explained in more detail in the Electronic Industries Association (EIA) 553 Cellular System specification. The cellular telephone uses a dotting sequence, the first segment of the message, to synchronize the cellular telephone hardware to a clock of the data message. A synchronization word (sync) indicates that the data sequence is about to start. Due to the unreliable nature of a typical terrestrial propagation channel, messages from a cell site are repeated multiple times. Repeat streams A and B include forty-bit words (which are defined in EIA 553), each word being repeated five times in the message. Each data word is approximately 4.4 msec "long" and an entire message (or frame), including the dotting sequence, sync word, and streams A, B, is approximately 46.3 msec long. The cellular telephone receives both of the data streams A, B but processes only one of them. The least significant digit of the unit's telephone number determines which one the data streams is processed. If the telephone number is even, stream A is processed; otherwise stream B is processed. However, to receive and process these data streams, the telephone's receiver must be on and drawing power the entire time, thereby reducing the time the unit can be used.
U.S. Pat. No. 5,175,874, Dec. 29, 1992, titled Radiotelephone Message Processing for Low Power Operation, discloses a process for reducing power consumption in a cellular telephone. The disclosed process receives, digitizes (i.e., quantizes to binary form), and stores a first data word. An error code in the data word is then checked to determine whether errors exist in the word. If there are errors, the digital word is corrected. A second data word is then received, digitized, checked for errors, and, error corrected if necessary. The second digital word is then compared with the first. If the words are not the same, the receiver remains on until at least two words are identical or the entire five-word message is received, whichever occurs first. If two received words are equal, the message is processed and the receiver is turned off during the remaining portion of the message, until the next synchronization word is received.
Thus, in the process disclosed in the above-cited patent, a minimum of two message words must be received, converted to binary form, error corrected, and compared to one another to determine whether they are identical. This process is therefore limited to systems that encode the message data with an error correction code. In addition, it is believed that the disclosed process provides an unacceptably high average number of words received (note that a minimum of two words must be received) and unacceptably high probabilities of miss and false acceptance.